The present invention relates to nonlinear processing of television signals, and more particularly, the present invention relates to nonlinear processing of low level television transition signals prior to passage through a medium subject to loss of the low level transition signals.
Recently introduced low-cast, consumer type video recorders are particularly sensitive to loss of picture details. These recorders, which have become widespread because of their low cost and light weight, suffer from bandwidth limitation as well as high noise and interference levels. In order to reduce objectionably high noise and levels, manufacturers introduced in the playback path a threshold circuitry which functioned as follows: High and low frequencies of the video signal were separated. The low frequencies were transmitted without modification. The high frequencies were then submitted to a threshold circuitry which allowed the signal to pass through only when above a predetermined value and were then recombined with the low frequencies to reconstitute the original video signal with a lower level of interference and noise. Unfortunately, the main drawback of that technique is a reduction or even a total elimination of desirable low level transitions. High level transitions are not modified. The result is a noise-free, interference free picture which is subjectively impaired: edges (large transitions) are readily quite visible, while fine picture details have completely disappeared. The resulting picture is objectionably too "cartoon"-like or "pasty-looking". Such picture defects cannot be corrected in playback since lost transitions cannot be recovered. Yet, before the present invention, limitations of the prior art technique of high frequency playback threshold would otherwise reestablish noise and interference at unacceptable levels.
Non-linear, level dependent manipulation techniques for television signals are not novel and have formed the subject matter of many prior patents. In general, such prior art disclosures described symmetrical systems having complementary effects on the input and output signals of a noisy or lossy medium; those systems consisted of two sections which could not operate separately. A number of prior approaches have been proposed for such symmetrical systems, including the provision of variable preemphasis, as shown in U.S. Pat. Nos. 3,117,278 and 3,207,854 to Johnson. A television broadcasting scheme including pretransmission preemphasis, and post reception deemphasis is set forth in U.S. Pat. No.3,286,024 to Melchior; that system followed classic preemphasis philosophy by preemphasizing the higher frequencies of the signal. One inventor, Ray Milton Dolby, has been active for a number of years in the area of noise reduction in communications systems, particularly audio recording. In one of his patents, U.S. Pat. No. 3,875,537, Dolby suggests the use of a threshold below which low level signals are provided with nonlinear processing, and above which signals are processed linearly. While Dolby made general reference to the suitability of video applications to nonlinear processors such as he described, he did not describe specific implementation of circuit architecture which could be utilized only prior to recording or transmission through a noisy and lossy medium, and which would result into an improved quality without further modification of the recording or transmitting device.